1. Field of the Invention.
The invention relates in general to the low temperature formation of metal oxides, including mixed metal oxides, and to the oxides so formed. In particular the invention relates to the formation of dense metal oxide films and nanoparticles, and to the films and nanoparticles so formed.
2. Description of the Prior art.
Previous workers have proposed to prepare metal oxides, and particularly metal oxide films, according to various procedures. In general, these prior expedients involved relatively high temperatures and harsh conditions that precluded practicing them to form such oxide coatings on substrates that tend to degrade at elevated temperatures. Thus, it was generally considered impossible to form metal oxide films on substrates such as, for example, temperature sensitive organic polymers, wood, paper, cotton, and the like. Deposition of oxides onto carbon and other oxidation sensitive substrates was also relatively ineffective due to elevated temperature oxidation and degradation of the substrate material during the deposition process. Previous expedients generally required the use of temperatures in excess of at least 300 and generally 500 degrees Centigrade.
Prior expedients generally did not produce fully dense metal oxide layers unless formed at or annealed at least 500 degrees Centigrade. Also, prior expedients did not produce pure metal oxide coatings. In general, some of the reactants were trapped in the coatings, which required the application of high temperatures to drive them off. Prior metal oxide forming procedures were often surface controlled in that the temperature of the surface upon which the oxide was to be formed was raised to a reaction temperature. The reactants reacted on or at the surface, driven by the elevated temperature of the surface, rather than in the vapor phase surrounding the surface. No reaction could take place without contact with the heated surface. This dictated that only temperature tolerant surfaces could be coated.
Metal oxide films, including mixed metal oxide films, enjoy application as hard surfaces, thermal and corrosion barriers, and the like. For a number of these applications, porosity in such films is highly undesirable because it provides a way for the environment to attack the underlying substrate. The production of fully dense films had generally been very difficult, if not impossible to achieve. For some applications, porosity or fractalization of the films can be beneficial, and a process that provides controllable morphology of the deposited oxides would, then, be of benefit. Metal oxide films generally suffered from the problem that they were very brittle. Thus, they tended to fracture and fail to provide the benefit for insoluble, and various expedients were resorted to in an effort to protect such coatings from conditions where their brittle nature would cause them to fail. This greatly limited the usefulness of such films or coatings.
It had generally been considered to be very difficult if not impossible to produce metal oxide nanoparticles, that is, particles having an average diameter of less than 100, and preferably less than 20 nanometers or even 15 nanometers. Such nanoparticles have very desirable properties for various applications, including, for example, compaction into various composite articles. They have extremely high surface to volume ratios that provide advantageous benefits as catalyst supports. Nanoparticles or nanocrystalline structures also possess unique mechanical properties that can provide advantageous performance.
Those concerned with these problems recognize the need for an improved low temperature metal oxide forming process, and the benefits to be derived from the products produced by such a process.